1. Field of the Invention
The present invention relates to a circuit for predicting a dead time, and more particularly, to a circuit for predicting a dead time during a charging operation on the capacitors.
2. Description of the Related Art
In the design of the power supply, the turn-on voltage drop on the rectifier has a great impact on the efficiency of the whole power supply, and further affects the reliability of the entire system. Therefore, synchronous rectifiers utilizing the power transistors driven by low voltage have gained more weight now.
FIG. 1 schematically shows a circuit diagram of a conventional synchronous rectifier. As shown in FIG. 1, the synchronous rectifier comprises a plurality of power transistors. The synchronous rectifier is operated by a Pulse-Width Modulation (PWM) signal provided by the system, cooperating with a driving signal received on a gate of the power transistor. However, if both the driving signal and the PWM signal are received by the power transistor at the same time, a shoot through phenomenon would occur on the power transistor. The transistor Q4 in FIG. 1 is taken as an example. Before the turn during the operation period, if the signal for driving the transistor Q4 is continuously output, the transistor Q4 would be turned on, thus a current would pass through the transistor Q4, leading to the shoot through phenomenon.
In order to avoid the shoot through phenomenon mentioned above, a set of circuit for predicting a dead time is additionally configured in the controller of the synchronous rectifier in order to calculate the time for turning off the driving signal. FIG. 2 schematically shows a circuit diagram of a conventional circuit for calculating the dead time. As shown in the diagram, a finite states machine (FSM) 101 receives a PWM signal PWM and anticipation settings input from the external. The operations of the FSM 101 and the counters 107, 109 are controlled by an oscillator 103. These two counters 107, 109 are configured to count a period of the PWM signal and a dead time, and also to output a counting result to the FSM 101. The FSM 111 controls a controller 111 based on the counting result received from the counters 107, 109, so that the controller 111 can generate a reset signal at the timing of the dead time. Therefore, the signal for driving the gate of the transistor Q4 is turned off before the PWM signal is turned on and after obtaining a time point by deducting the dead time from the PWM period. When the reset signal is received by the synchronous rectifier, the driving signal is immediately turned off, thus it is possible to turn off the driving signal before the next PWM period begins, such that the driving signal no longer overlaps with the PWM signal and the shoot through phenomenon mentioned above can be prevented.
However, if the period of the PWM signal is calculated by the counter, usually a high frequency oscillator requiring a higher power is used to drive the counter, which would consume great power for the whole system.